spi: sunxi: set maximum and minimum speed of SPI master

The speed limits are unset in the sun4i and sun6i SPI drivers.

The maximum speed of SPI master is used when maximum speed of SPI slave
is not specified. Also the __spi_validate function should check that
transfer speeds do not exceed the master limits.

The user manual for A10 and A31 specifies maximum
speed of the SPI clock as 100MHz and minimum as 3kHz.

Setting the SPI clock to out-of-spec values can lock up the SoC.

Signed-off-by: Michal Suchanek <hramrach@gmail.com>
--
v2:
new patch
v3:
fix constant style
Signed-off-by: Mark Brown <broonie@kernel.org>